Magnetic devices, such as DC to DC power supplies, isolation transformers, voltage step down transformers, and inductors are needed in various electronic circuits. It is time consuming and difficult to insert discrete packaged transformer and inductor components onto a printed circuit board (i.e., PCB), and the high profile of these parts is not compatible with many types of finished products, such as cell phones, personal digital assistants (i.e., PDAs), and notebook computers. Further, the wave soldered thru hole connections, or the surface mount soldering pads, required for attaching these transformers or inductors to the PCB provide further opportunities for the introduction of fatal manufacturing defects, especially with current PCBs typically having as many as 20 conductive layers.
To solve the discrete component problem for transformers and inductors on PCBs discussed above, it is known to use the various individual conductor layers that comprise a typical PCB to create planar electrical coils through which electrical currents are propagated to create a magnetic field. By stacking such coils, for example one coil per conductor layer of the PCB, one on top of the other, and by connecting the individual coils together by means of what are known as vias in the PCB, it is possible to create stacked inductive coils having reasonably small size and sufficient total inductance values.
By interleaving the coils and separating the electrical connections into two groups, one group, for example the odd numbered layers, as a primary winding, and the second group, for example the even numbered layers, as the secondary winding, then the stacked inductors may be formed into a power transformer. To illustrate the transformer formation with a ten to one step down voltage transformation, the first layer would be a primary ten turn winding, the second layer would be a secondary one turn winding, the third layer would be another primary ten turn winding, and so on through as many of the PCB layers as are desired to achieve the necessary current and magnetic field, or until the last available PCB layer is reached.
However, there is a problem with the planar coils described above. The need to make electrical connections through the insulating layers separating the conductive layers of the PCB disrupts the direction and flow of electrical current and creates what is known as leakage inductance. Leakage inductance reduces the magnetic coupling between windings of the transformer, and increases thermal management problems and consequently reduces component lifetime. The need to make layer to layer contact also results in increased length of the conductor runs, and consequent increased winding resistance, and thus again increased thermal management problems.
Therefore, a problem exists with efficiently making contact between planar coils, and in laying out the coils to maximize the layer to layer overlap, and thus the total inductance, while minimizing the total coil length and the number of contacts outside the magnetic field area.
An apparatus and method for providing planar magnetic fields for PCB inductance and voltage transformation is presented having a typical PCB with multiple conductive layers electrically separated by insulating layers. A set of primary windings and secondary windings having a specified order are arranged on the layers of the PCB to form the magnetic device. In a preferred embodiment of the invention, a first primary winding is created on a first one of the conductive layers of the PCB, a first secondary winding is created on a second conductive layer of the PCB directly below the first winding. A second primary winding is created on the third conductive layer of the PCB, and a second secondary winding is created on a fourth conductive layer of the PCB, continuing in this fashion until each one of the desired number of primary and secondary windings are created within a PCB.
In another embodiment of the invention, the odd numbered primary windings spiral inward toward a core region in either a clockwise or counter clockwise direction, and the even numbered primary windings spiral outward from the core region in the same direction as the odd numbered primary windings. Also the odd numbered secondary windings spiral inward toward the core region, and the even numbered secondary windings spiral outward from the core region in the same direction as the odd numbered secondary windings. In an alternate embodiment of the invention, the odd numbered secondary windings spiral outward from the core region rather than inward, and the even numbered secondary windings spiral inward.
In a further embodiment of the invention, the number of conductive PCB layers is smaller than the needed number of primary and secondary windings. The remaining number of required windings are created on one or two small multilayered PCBs, which are attached to the surface of the PCB, typically directly above or below the PCB winding, in order to continue the coil stack.
In another embodiment of the invention, all of the electrical connections between coils on different PCB layers are made inside the magnetic field area of the coil stack to reduce leakage inductance.
In yet another embodiment of the invention, the odd numbered ones of the coils are connected together in series to form the primary windings of a transformer, and the even numbered ones are electrically connected together in parallel to form the secondary windings of the transformer. In an alternate embodiment the secondary windings are single turn coils.
A PCB winding technique is presented that minimizes the number and length of layer to layer interconnections, and provides the interconnections inside the magnetic field of a magnetic device built on a PCB. This improves the high parasitic losses found in present planar magnetic winding techniques. The improvement is due to better usage of the area of conductive material (i.e., Copper) inside the magnetic field area since less conductive material is used for layer to layer interconnection. This is accomplished by spiraling the first layer inward towards the magnetic core, then the next interconnected layer down spirals outward away from the core, etc. By making all of the magnetic connections inside the magnetic field area, the usual leakage inductance is minimized, and reducing the overall length of the windings by not leaving the magnetic field area to make connects, the electrical resistance of the winding is reduced, thereby reducing parasitic current loss and unwanted component heating.